Process and apparatus for converting thermal energy into electrical energy



April 1954 G. N. HATSOPOULOS ETAL 3,129,345

PROCESS AND APPARATUS FOR CONVERTING THERMAL ENERGY INTO ELECTRICAL ENERGY Filed Nov. 5. 1959 /6 F/ 6. 2 f M 8 W FIG. 3

6 N 8 b 5 U w H- 12 m .8 o INVENTORS Mama-,6 01/7/ 07 G0GMAMTJ0P00L0$ BY Ja/w PS'AI007HAl/S Arrae/u'r United States Patent cc 3,129,345 PROCESS AND APPARATUS FOR CONVERT- ING THERMAL ENERGY INTO ELECTRICAL ENERGY George N. Hatsopoulos, Lexington, and John Psaronthaids, Arlington, Mass assignors to Thermo Electron Engineering tjorporation, a corporation of Delaware Filed Nov. 5, 1959, Ser. No. 851,124 15 Claims. (Cl. 3104) This invention relates to the conversion of thermal energy into electrical energy, and more particularly to process and apparatus for effecting such conversion, which apparatus does not require any moving mechanical parts for so doing. The apparatus will be hereinafter referred to as a thermoelectron engine.

In co-pending application Serial No. 723,336, filed March 24, 1958 is disclosed a thermoelectron engine for converting thermal energy from any available source into electrical energy by employing the thermal energy to heat one of two parallel electron emissive surfaces (which heated surface, for convenience, is hereinafter referred to as the emitter or hot surface), spaced from the other surface (hereinafter referred to as the collector or cooler surface) a distance not more than about 0.002 inch, preferably not more than about 00005 inch. The emitter is heated to a temperature of at least 1800 F., desirably within the range of from 1800" to 4000 F., preferably from 2000 to 3000 F., the temperature to which it is heated depending chiefly on the source of heat available and its material of construction. Desirably the cooler surface is maintained at least 350 F. below the temperature of the hot surface, preferably from 400 to 500 F. below the temperature of the hot surface. Its temperature may be from 85 to 1500 F.

The present invention is an improvement on the process and apparatus disclosed in said co-pending application, which improvement results in a surprising increase in efliciency.

In accordance with the present invention, an emitter electron emissive surface and a collector electron emissive surface are maintained under vacuum and at least one intermediate electron emissive surface is interposed between these two, the spacing between each electron emissive surface and the next surface facing it being not greater than 0.002 inch, preferably not greater than 0.0005 inch. The emitter is heated to a temperature at least 350 F. higher than the temperature of the collector, preferably from 400 to 500 F. higher. The emitter has a work function of not greater than approximately 15 kT in which expression k is Boltzmanns constant (0.861 volts per degree Kelvin) and T is the maximum temperature in degrees Kelvin to which the emitter is heated in use. The intermediate electron emissive surface or surfaces and the collector have work functions not greater than that of the emitter. There is thus established a flowing stream of electrons from the emitter to the intermediate electron emissive surface herein referred to as a combined emitter and collector, and where more than one such combined emitter and collector are used, from each such combined emitter and collector to the next, which stream reaches the collector. The emitter and the collector are provided with conductors in circuit with a load through which conductors current is taken off.

The number of intermediate electron emissive surfaces can be varied as desired. In general, the number of such surfaces should not exceed 4; 1 or 2 such intermediate electron emissive surfaces are preferred.

It is important that the spacing between the emitter and the face of the intermediate surface contiguous to the emitter and spacing between adjacent intermediate electron emissive surfaces, as well as the spacing between 3,129,345. Patented Apr. 14, 1964 the electron emissive surface opposite to the collector and the collector be not more than 0.002 inch, preferably not more than 0.0005 inch. The desired spacing between the emitter and the intermediate combined emitter and collector contiguous thereto and between adjacent intermediate combined emitters and collectors and between the last of the series of intermediate combined emitters and collectors and the collector is obtained by employing corundum, particularly ruby or sapphire, spheres or by suitable ceramic spacers.

The surfaces should be parallel to each other and main tained under a vacuum of at least 5 X 10* mm. of mercury. When an inert gas such as helium, krypton, argon, neon, xenon or a mixture of these gases is present, the vacuum may be of the order of 10- mm. of mercury or lower.

A preferred embodiment of this invention involves process and apparatus for converting thermal energy into electrical energy by maintaining under vacuum an emittter and collector as hereinabove described with at least one electron emissive member interposed between the emitter and collector, the interposed member or members having electron emissive material on the opposite sides thereof and being spaced from the immediately adjacent surfaces on the opposite sides thereof a distance not more than about 0.002 inch, preferably not more than 0.005 inch. The emitter is heated to a temperature of at least 1800 F., desirably to a temperature of from 1800 to 4000 F., preferably from 2000 to 3000 F., while the collector is maintained at a temperature of from 400 to 500 F. below the temperature of the emitter, preferably at a temperature within the range of from to 1500 F. and the interposed member or members are maintained at an equilibrium temperature between that of the emitter and collector. Flow of a stream of electrons takes place from the emitter to and through the intermediate members to the collector. Current is taken off from the emitter and collector through suitable metallic leads.

Surprisingly an increase in efiiciency of about 30% takes place as compared with a thermoelectron engine involving the emitter and collector as disclosed, for example, in the aforesaid co-pending application Serial No. 723,336, not having such interposed member or members. While the reason for this surprising improvement in efiiciency is not fully known, it is believed to be due chiefly to reduction in radiation losses effected by the presence of the intermediate member or members.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accom panying drawing showing, for purposes of exemplification, preferred forms of this invention without limiting the claimed invention to such illustrative instances and in which:

FIGURE 1 is a vertical section through a thermoelectron engine involving a single intermediate combined emitter and collector and emboding the present invention;

FIGURE 2 is a diagrammatic showing of a thermoelectron engine involving two intermediate members between the emitter and collector; and

FIGURE 3 is a graph showing the efliciency of the themoelectron engine of FIGURE 1 as compared 'with an engine in which the emitter and collector are of the same electron emissive material but not having an intermediate combined emitter and collector, i.e. not embodying the present invention but otherwise truly comparative.

Referring to FIGURE 1 of the drawing, 10 is a housing of stainless steel, ceramic material or other suitable material of construction suitably sealed by a base plate 11. The housing is provided with an evacuating duct 12 for evacuating the housing. Duct 12. is provided with a suitable seal or valve closure to maintain the housing under the desired vacuum once it has been evacuated.

Disposed within housing is a casing 13 designed to contain a suitable source of heat, for example, a heating element, chemicals which react exothermical-ly, or radioactive isotopes. The emitter 14 constitutes the base of the casing 13, this emitter being disposed in direct heat exchange relation with the source of heat within casing .13.

Radiation shield r15 concentric with casing 13 is positioned within housing 10 and serves to support the combined emitter and collector 16 between the emitter 14 and the collector 17. Spacers 18, desirably corundum spheres, are positioned in depressions 19 in the combined emitter and collector 16. Three equi-spaced spacers 18 may be so positioned in the top surface of the combined emitter and collector 16. Each of these spacers is of a diameter such that when disposed in the depressions '19, the emitter 14 is spaced from the combined emitter and collector 16 a distance not greater than 0.002 inch, preferably not greater than 0.0005 inch.

In like manner, the underside of the combined emitter and collector 16 is spaced from the upper surface of the collector 17 by spherical spacers 21 positioned in depressions 22 in the face of the collector 17. Collector 17 is suitably supported on the base plate 11 by means of the supports 23 in threaded engagement with the base plate 11. A radiation shield 24, concentric with the radiation shield '15, is secured to the collector 17 by the securing nuts 25 threaded onto the threaded extensions 26 of the supports 23. Suitable spacer members 27 of insulating material, desirably ceramic, pass through the side walls of the radiation shield 15 and engage the casing 13 to permit adjustment of the position of the emitter relative to the collector 17 and the combined emitted and collector 16. Similar adjusting screws 28 of insulating material, desirably ceramic, are provided in the high temperature shield 24 to adjust the position of the combined emitter and collector 16 and maintain this element in adjusted position. A further adjusting spacer 29 engages the top of casing 13 exerting pressure thereon to maintain the emitter 14 in desired spaced relation relative to the upper surface of the interposed combined emitter and collector 16. Instead of having these members 27, 28 and 29 of insulating material, the tips only, which are in engagement with the casing 13, the outer Wall of the radiation shield 15, and the top of the casing 13, respectively, may be made of ceramic material.

The radiation shields 15 and 24 may be of molybdenum or tungsten; the low thermal emissivities of these materials allow them to act as good back radiators of heat.

A conductor 31 leads from the casing =13 which is in electrical contact with the emitter 14 through suitable packing 32 to the exterior of the housing 10. A second conductor 33 leads from the getter 34 through the seal 32 to the exterior of the housing. The other end of the getter 34 is grounded on thebase plate 11. The collector 17 is also electrically connected with the base plate via support 23.

Getter 34 may be of any well-known material capable of absorbing and/or adsorbing gases such, for example, as barium or tantalum, to maintain the vacuum within housing 10, i.e. to absorb and/or absorb any gases which may be liberated within the housing and thus maintain the desired vacuum.

To provide a gas tight seal, a copper or other suitable gasket 35 is positioned between the base plate 11 and the flange 36 of housing 10.

The emitter 14, collector I17 and inter-mediate member 16, while shown in FIGURE 1 as discs, may be of any desired shape including cylindrical, spherical or conical shapes spaced apart the desired distance by suitable spacers of low thermal conductivity, high mechanical strength and good electrical insulating properties, e.g.

corundum, particularly ruby or sapphire, or ceramic, which make point contact with the electron emissive surfaces under pressure to maintain the desired close spaced relationship at all times during steady-state operation. These members desirably are about A; to inch thick and when circular plates are employed, have a diameter of 2 to 4 inches. In the case of thinner plates, the diameter is less and for thicker plates, the diameter may be greater, i.e. 4 inches for a A inch thick plate and 2 inches for a /8 inch thick plate. It will be understood the above dimensions are the optimum for presently available electron emissive materials and this invention is not to be limited to these dimensions.

The combined emitter and collector 16 should be of electron emissive material throughout its thickness. For example, it may consist of Philips cathodes, both A and B types, i.e. sintered porous tungsten impregnated with various oxides, usually in the form of carbonates, which carbonates upon activation are converted to oxides. Type A cathodes contain barium oxide and aluminum oxide in the mole ratio of five to two. Type B cathodes contain barium oxide, aluminum oxide and calcium oxide in the mol ratio of five to two to three. For the combined emitter and collector porous tungsten is impregnated with these oxides to provide oxide electron emissive surfaces on the opposite sides of the disc or plate.

The combined emitter and collector, as well as the emitter 14 and the collector 17, may be of any of the following electron emissive materials:

(a) Thoriated tungsten;

(b) Tungsten coated with casium;

(c) Thoria, i.e. ceramic ThO (d) Barium oxide, strontium oxide, calcium oxide, and mixtures of these oxides;

(e) Molybdenum housing or stocking filled with granules of a fused barium oxide and aluminum oxide mixture;

(f) Lanthanum oxide (La O (g) Perforated molybdenum sleeve or housing containing sintered thorium oxide; and

(h) Pure tungsten.

The collector 1'7 and the combined emitter and collector 16 may be a composite cathode consisting of silver with a monolayer of oxygen and a monolayer of cesium on the oxygen layer on each of the sides thereof in the case of the combined emitter and collector. The collector may have such monolayers on only the side facing the emitter. Since the technique of producing such composite cathodes is known, further description thereof is believed unnecessary.

The present order of preference for maximum efiiciency are the combinations given in the table which follows:

Table Emitter Combined Emitter and Collector Collector WRIEDUIHK 1. Type A cathodes as 2. Type A cathodes as defined above.

. Tungsten cathodes with adsorbed cesium.

. Type A cathodes as defined above.

. Type B cathodes as defined above.

. Type B cathodes as defined above. Type B cathodes as defined above.

Thoriated tungsten- Type B cathodes as defined above.

Tungsten cathodes with adsorbed cesium.

Type B cathodes as defined above.

Cesium adsorbed on tungsten.

Type B cathodes as defined above.

Oxide coated nickel Cesium adsorbed on a tungsten surface.

Composite cathode.

Type B cathodes as defined above.

Cesium adsorbed on tungsten.

Do. Type B cathodes as defined above. Oxide coated nickel.

tion of the electron emissive surface at such temperature will not materially reduce their life. The hot electron emissive surface or surfaces may be heated by any desired mode of heat transfer, i.e. radiation, conduction, convection or condensation, or by any combination of two or more of these methods of heat transfer.

FIGURE 2 shows diagrammatically a construction embodying the present invention in which two intermediate members are interposed between the emitter 14 and the collector 17. It will be appreciated that in this figure, as well as in FIGURE 1, the parts are not shown to scale because in View of the minute spacing between the emitter and the intermediate members, it is not possible to do so. In FIGURE 2 intermediate members 41 and 42 are positioned between the emitter 14 and the collector 17 with their electron emissive surfaces on the opposite sides thereof spaced from the immediately contiguous electron emissive surface a distance not greater than 0.002 inch, preferably not greater than 0.0005 inch. Current is drawn from the unit via the connections 45 and 46 which pass through suitable seals in the evacuated housing 47 to the load 48.

In operation the interior of the housing (FIGURE 1) is evacuated through duct 12. The adjusting bolts 27, 23 and are positioned so that the emissive surfaces will be centered (aligned) with respect to each other. The emitters are subjected to an activation temperature which depends on the emissive material used, usually about the same temperature or somewhat higher than the temperature to which the emitter is heated, in operation. Heating at this temperature takes place for only a few minutes and insures the removal of substantially all gases and vapors from the electron emissive material. Activation is continued until no more gases escape from the emitters or collectors. It is carried out under a vacuum of from about 5 10 to 5X10 mm. of mercury. Higher vacuum can, of course be used but is not necessary and hence it is wasteful to employ higher vacuums. The time of activation will depend on the electron emis sive materials employed, the vacuum conditions maintained during the activation and the spacing between the hot and cooler emissive surfaces. It may be accomplished in about three hours or longer. Where close spacing of the hot and cooler electron emissive surfaces is maintained, the evacuation may require a weeks time and even longer. It will be appreciated that the spacing between each pair consisting of a collector and an emitter need not be increased during activation, but by so doing the activation time is greatly reduced and complete activation is insured. During the activation, emitted gases and vapors fiow through the duct 12 which communicates with a vacuum pump or other suitable source of vacuum.

After the activation has been completed, evidenced by the cessation of gas emission from electron emissive surfaces, duct 12 is sealed when the interior of housing 10 has been placed under the desired vacuum. Steady-state operation then commences. During this operation the emitter 14 heated by thermal energy emits electrons which flow to the combined emitter and collector 16 and thence to the collector 17. Current is taken off through the conductors 31, 33 in circuit with a load (not shown)- FIGURE 3 is a graph of voltage output versus efficiency. The upper curve 50 is a plot of efiiciency (the ratio of power output to heat input) versus the voltage output on the basis of actual operation of the thermoelectron engine of FIGURE 1. Curve 60 is a similar plot of a comparative thermoelectron engine which differs from that of FIGURE 1 in the omission of the combined emitter and collector 16. The dimensions, temperature conditions, vacuum maintained and electron emissive materials used were the same. It will be noted from FIGURE 3 that for a voltage output of .8 volt, where the thermoelectron engine has its maximum efiiciency, the engine of FIGURE 1 has an efiiciency of slightly less than 16% whereas the comparative engine not containing the intermediate member 16 has an efliciency of slightly more than 12%. Thus the present invention results in an increase in efliciency of the order of 30%.

A number of thermoelectron units, each consisting of an emitter, collector and one or more combined intermediate emitters and collectors, such as shown in FIG- URES l and 2, may be arranged in cascade within a single housing maintained under vacuum and connected in series so that the voltage generated in one unit builds up on the next, etc. of the assembly, until the last unit of the assembly is reached, thus providing any desired voltage, within reason.

Since dilferent embodiments of the invention could be made without departing from the scope of this invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process of converting thermal energy into electrical energy comprising maintaining under vacuum two electron emissive surfaces parallel to each other, one of said surfaces being an emitter and the other a collector, with at least one electron emissive member interposed between the emitter and collector, said interposed member having electron emissive material on the opposite sides thereof and being spaced from the immediately adjacent electron emissive surfaces on the opposite sides thereof a distance not more than 0.002 inch, heating the emitter to a temperature of at least 350 F. higher than that of the collector and maintaining said interposed member at an equilibrium temperature between the temperature of the emitter and that of the collector, and taking off current from the emitter and collector.

2. A process of converting thermal energy into elec trical energy comprising maintaining under vacuum two electron emissive surfaces parallel to each other, one of said surfaces being an emitter and the other a collector, the emitter having a work function not greater than approximately 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated the collector having a work function not greater than that of the emitter, with at least one electron emissive member interposed between the emitter and collector, said interposed member having electron emissive material on the opposite sides thereof and being spaced from the immediately adjacent electron emissive surfaces on the opposite sides thereof a distance not more than 0.002 inch, heating the emitter to a temperature of at least 350 F. higher than that of the collector and maintaining said interposed member at an equilibrium temperature between the temperature of the emitter and that of the collector, and taking off current from the emitter and collector.

3. A process of converting thermal energy into electrical energy comprising maintaining under vacuum two electron emissive surfaces parallel to each other, one of said surfaces being an emitter and the other a collector, the emitter having a work function not greater than approximately 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, the collector having a work function not greater than that of the emitter, with at least one electron emissive member interposed between the emitter and collector, said interposed member having electron emissive material on the opposite sides thereof and being spaced from the immediately adjacent electron emissive surfaces on the opposite sides thereof a distance not more than 0.0005 inch, heating the emitter to a temperature of from 400 to 500 F. higher than that of the collector and maintaining said interposed member at an equilibrium temperature between the temperature of the emitter and that of the collector, and taking off current from the emitter and collector.

4. A process of converting thermal energy into electrical energy comprising maintaining under vacuum two electron emissive surfaces parallel to each other, one of said surfaces being an emitter and the other acollector, the emitter having a work function not greater than approximately kT in which expression It is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, the collector having a work function not greater than that of the emitter, with at least one electron emissive member interposed between the emitter and collector, said interposed member having electron emissive material on the opposite sides thereof and being spaced from the immediately adjacent electron emissive surfaces on the opposite sides thereof a distance not more than 0.0005 inch, heating the emitter to a temperature of at least 1800 F., maintaining the collector at a temperature of from 400 to 500 F. below the temperature of the emitter, and the interposed member at a temperature between the temperature of the emitter and that of the collector, and taking off current from the emitter and collector.

5. A process of converting thermal energy into electrical energy comprising maintaining under vacuum two electron emissive surfaces parallel to each other, one of said surfaces being an emitter and the other a collector, the emitter having a work function not greater than approximately 15 k1" in which expression k is Boltzmanns constant and T is the maximtun temperature in degrees Kelvin to which the emitter is heated, the collector having a work function not greater than that of the emitter, with at least one electron emissive member interposed beween the emitter and collector, said interposed member having electron emissive material on the opposite sides thereof and being spaced from the immediately adjacent electron emissive surfaces on the opposite sides thereof a distance not more than 0.0005 inch, heating the emitter to a temperature of from 1800 to 4000 F., maintaining the collector at a temperature of from 400 to 500 F. below the temperature of the emitter, and the interposed member at an equilibrium temperature between the temperature of the emitter and that of the collector, and taking off current from the emitter and collector.

6. A process of converting thermal energy into electrical energy comprising maintaining under vacuum an emitter electron emissive surface, a collector electron emissive surface, and at least one intermediate member positioned between said emitter and collector parallel thereto having electron emissive material on the opposite sides thereof defining a pair of intermediate electron emissive surf-aces, each of said intermediate surfaces being spaced from contiguous electron emissive surfaces a distance not more than about 0.002 inch, said emitter having a work function not greater than approximately 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, each of said other electron emissive surfaces, including the collector, having a work function not greater than that of the emitter, heating the emitter to a temperature at least 350 F. higher than that of the collector, maintaining said intermediate surfaces between the emitter and collector at a temperature between the temperature of the emitter and collector, and thus establishing a flow of electrons from said emitter to and through said intermediate surfaces to the collector, and taking off current from the emitter and collector.

7. A process of converting thermal energy into electrical energy comprising maintaining under vacuum an emitter electron emissive surface, a collector electron emissive surface, and at least one intermediate member positioned between said emitter and collector parallel thereto having electron emissive material on the opposite sides thereof defining a pair of intermediate electron emissive surfaces, each of said intermediate surfaces being spaced from contiguous electron emissive surfaces a distance not more than about 0.0005 inch, said emitter having a work function not greater than approximately 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, each of said other electron emissive surfaces, including the collector, having a work function not greater than that of the emitter, heating the emitter to a temperature of from 400 to 500 F. higher than that of the collector, maintaining said intermediate surfaces between the emitter and collector at a temperature between the temperature of the emitter and collector, and thus establishing a flow of electrons from said emitter to and through said intermediate surfaces to the collector, and taking off current from the emitter and collector.

8. A process of converting thermal energy into electrical energy comprising maintaining under vacuum an emitter electron emissive surface, a collector electron emissive surface, and at least one intermediate member positioned between said emitter and collector parallel thereto having electron emissive material on the opposite sides thereof defining a pair of intermediate electron emissive surfaces, each of said intermediate surfaces being spaced from contiguous electron emissive surfaces a distance not more than about 0.005 inch, said emitter having a work function not greater than approximately 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, each of said other electron emissive surfaces, including the collector, having a work function not greater than that of the emitter, heating the emitter to a temperature within the range of from 1800 to 4000 F. while maintaining the collector at a temperature within the range of from to 1500 F., maintaining said intermediate surfaces between the emitter and collector at a temperature between the temperature of the emitter and collector, and thus establishing a flow of electrons from said emitter to and through said intermediate surfaces to the collector, and taking off current from the emitter and collector.

9. A process of converting thermal energy into electrical energy comprising maintaining under vacuum an emitter electron emissive surface having a work function not greater than 15 F1], in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, a collector electron emissive surface parallel to said emitter and spaced therefrom and having a work function not greater than that of the emitter, and a plurality of intermediate electron emissive members between said emitter and collector parallel to each other and to the emitter and collector, and each having electron emissive material on the opposite sides thereof, having a work function not greater than that of the emitter, and each having an electron emissive surface spaced from an immediately contiguous electron emissive surface a distance not more than about 0.002 inch, heating the emitter to a temperature of at least 350 F. higher than that of the collector, maintaining said intermediate electron emissive surfaces at an equilibrium temperature between the temperature of the emitter and collector and thus establishing a flow of electrons from said emitter to and through said intermediate electron emissive surfaces to the collector, and taking off current from said emitter and collector.

10. A process of converting thermal energy into electrical energy comprising maintaining under vacuum an emitter electron emissive surface having a work function not greater than 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, a collector electron emissive surface parallel to said emitter and spaced therefrom and having a work function not greater than that of the emitter, and a plurality of intermediate electron emissive members between said emitter and collector parallel to each other and to the emitter and colc o a each having electron emissive material on the opposite sides thereof, having a work function not greater than that of the emitter, and each having an electron emissive surface spaced from an immediately contiguous electron emissive surface a distance not more than about 0.0005 inch, heating the emitter to a temperature within the range of from 1800 to 4000" F. while maintaining the collector at a temperature of from 400 to 500 F. below the temperature of the emitter, maintaining said intermediate electron emissive surfaces at an equilibrium temperature between the temperature of the emitter and collector and thus establishing a flow of electrons from said emitter to and through said intermediate electron emissive surfaces to the collector, and taking off current from said emitter and collector.

1 1. A process of converting thermal energy into electrical energy comprising maintaining under vacuum an emitter electron emissive surface having a work function not greater than kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, a collector electron emissive surface parallel to said emitter and spaced therefrom and having a Work function not greater than that of the emitter, and a plurality of intermediate electron emissive members between said emitter and collector parallel to each other and to the emitter and collector, and each having electron emissive material on the opposite sides thereof, having a work function not greater than that of the emitter, and each having an electron emissive surface spaced from an immediately contiguous electron emissive surface a distance not more than about 0.0005 inch, heating the emitter to a temperature of from 2000 to 3000 F. while maintaining the collector at a temperature Within the range of from 85 to 1500 F., maintaining said intermediate electron emissive surfaces at an equilibnium temperature between the temperature of the emitter and collector and thus establishing a flow of electrons from said emitter to and through said intermediate electron emissive surfaces to the collector, and taking off current from said emitter and collector.

12. Apparatus for converting thermal energy into electrical energy comprising, in combination, a housing under vacuum, an emitter electron emissive surface having a Work function not greater than 15 kT in which expression k is Boltzmanns constant and T is the maximum temperature in degrees Kelvin to which the emitter is heated, a collector electron emissive surface having a work function not greater than that of the emitter, spaced from and parallel to said emitter, at least one member between said emitter and collector having electron emissive material on the opposite sides thereof, spaced from an immediately adjacent electron emissive surface a distance not greater than 0.002 inch, means for heating the emitter while maintaining the collector at a temperature below that of the emitter and the said intermediate member at an equilibrium temperature between that of the emitter and the collector, and means for taking off current from said emitter and collector.

13. Apparatus for converting thermal energy into electrical energy comprising, in combination, a housing under vacuum, an emitter electron emissive surface, a collector electron emissive surface, spaced from and parallel to said emitter, at least one member positioned between said emitter and collector parallel thereto, having electron emissive material on the opposite sides thereof and being spaced from an immediately adjacent electron emissive surface a distance not greater than 0.002 inch, means for heating the emitter While maintaining the collector at a temperature below that of the emitter and the said intermediate surface at an equilibrium temperature between that of the emitter and the collector, and means for taking off current from said emitter and collector.

14. The apparatus for converting thermal energy into electrical energy as defined in claim 13, in which the emitter is sintered porous tungsten impregnated with barium oxide and aluminum oxide, the collector is a tungsten surface having cesium adsorbed thereon, and the said intermediate members are constituted of porous tungsten impregnated with barium oxide and aluminum oxide.

15. Apparatus for converting thermal energy into electrical energy comprising, in combination, a housing under vacuum, an emitter electron emissive surface in said housing, a collector electron emissive surface parallel to and spaced from said housing, a plurality of electron emissive surfaces positioned between said emitter and collector parallel thereto, each of said surfaces being spaced from the immediately contiguous electron emissive surface a distance not more than 0.0005 inch, means for heating the emitter while maintaining the collector at a lower temperature than the emitter and maintaining said intermediate electron emissive surfaces at a temperature between that of the emitter and collector, and means for taking 011? current from said emitter and collector.

References Cited in the file of this patent UNITED STATES PATENTS 2,510,397 Hansell .Tune 6, 1950 2,581,446 Robinson Jan. 8, 1952 2,863,074 Johnstone Dec. 2, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.- 3,129,345 April 14L I964 George No Hatsopoulos et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2. line 26, for "0,005" read 0,0005

column 3, line 36 for "emitted" read emitter line 64 for "absorb" read adsorb column 4, line 21 for "mole" read moI line 31 for "casium read M cesium column 8, line 24, for "0005" read M 00005 a Signed and sealed this 4th day of August 1964,

' (SEAL) Attest:

ERNEST w. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.- 3,l29,345 April 14 1964 George N; Hatsopoulos et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 26, for "0.005" read 0,0005

column 3, line 36, for "emitted" read emitter line 64 for "absorb" read adsorb column 4, line 21, for "mole" read mol line 31 for "casium read cesium column 8, line 24, for 0.005" read 0.0005

Signed and sealed this 4th day of August 1964c (SEAL) ERNEST w SWIDER Attest:

EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A PROCESS OF CONVERTING THERMAL ENERGY INTO ELECTRICAL ENERGY COMPRISING MAINTAINING UNDER VACUUM TWO ELECTRON EMISSIVE SURFACES PARALLEL TO EACH OTHER, ONE OF SAID SURFACES BEING AN EMITTER AND THE OTHER A COLLECTOR, WITH AT LEAST ONE ELECTRON EMISSIVE MEMBER INTERPOSED BETWEEN THE EMITTER AND COLLECTOR, SAID INTERPOSED MEMBER HAVING ELECTRON EMISSIVE MATERIAL ON THE OPPOSITE SIDES THEREOF AND BEING SPACED FROM THE IMMEDIATELY ADJACENT 